[Bug 44895] League Of Legends: Unable to log in if behind OpenWRT based router

[wine-bugs at winehq.org]

https://bugs.winehq.org/show_bug.cgi?id=44895

--- Comment #11 from jaapbuurman at gmail.com ---
I found out the source of the problem. For some reason, Riot's server doesn't
accept fragmented packets. On Windows, the League of Legends launcher sends out
a large packet with a Don't Fragment (DF) flag set, which forces the path MTU
discovery protocol to tone down the MTU. As can be seen in the following packet
capture:

08:40:57.627787 IP (tos 0x0, ttl 128, id 13958, offset 0, flags [DF], proto TCP
(6), length 2731)
    192.168.1.154.49972 > 104.16.21.24.443: Flags [P.], cksum 0x4a08 (incorrect
-> 0xa8db), seq 603:3294, ack 3193, win 255, length 2691
08:40:57.628062 IP (tos 0xc0, ttl 64, id 59635, offset 0, flags [none], proto
ICMP (1), length 576)
    192.168.1.1 > 192.168.1.154: ICMP 104.16.21.24 unreachable - need to frag
(mtu 1492), length 556
    IP (tos 0x0, ttl 128, id 13958, offset 0, flags [DF], proto TCP (6), length
2731)


However, on Wine, the same large packet does not have the DF flag set:

08:54:48.047494 IP (tos 0x0, ttl 64, id 45537, offset 0, flags [none], proto
TCP (6), length 2731)
    192.168.1.153.56746 > 104.16.20.24.443: Flags [P.], cksum 0x4907 (incorrect
-> 0x2378), seq 603:3294, ack 3193, win 278, length 2691


So in this case, the packet simply gets fragmented and the router doesn't let
the PC know it needs to tone down the MTU. For people like me, that have their
LAN with the standard MTU of 1500 and an internet connection over PPPOE with an
effective MTU of 1492, this is an issue. The subsequent packets are then send
with an MTU 1500, fragmented because the WAN interface only does 1492, and
Riot's servers do not like that. For people that also have a MTU of 1500 on the
WAN interface, this bug doesn't trigger.

A workaround is to simply decrease the MTU on the LAN interface on the Linux
machine to same value as the MTU on the WAN interface of the router, in my case
1492. In order to fix this bug, Wine should set the DF flag on the large packet
so that it can discover the correct MTU setting automatically.

For a full discussion of this bug plus better formatted packet captures, please
view the following topic:
https://forum.lede-project.org/t/solved-strange-wine-league-of-legends-lede-bug/14114/

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